A conventional diaphragm for a loudspeaker (hereinafter referred to as “diaphragm”) is demonstrated hereinafter with reference to FIGS. 5 through 7. FIG. 5 is a half sectional side view of the conventional loudspeaker. FIG. 6 is a half sectional side view of the conventional diaphragm. FIG. 7 is a plan view of the conventional diaphragm shown from its bottom.
As shown in the drawings, magnetic circuit 5 is formed of yoke 2, disk shaped magnet 3 and top plate 4. Yoke 2, which is made of magnetic material, has a cylindrical outer wall and protrudes a center of its bottom upward.
Magnetic gap 5a is formed between a circular inside of the outer wall of yoke 2 and an outer circumference of top plate 4. Neodymium or ferrite base magnet is generally used as magnet 3.
Resin frame 7 is coupled with an outer circumference of yoke 2. A known means such as adhesive, press fitting or outsert molding to resin frame 7 is used as the coupling with yoke 2.
Cone shaped diaphragm 1, which is formed of main body 1a of the diaphragm and edge 1b of an outer circumference, is formed by resin-molding with its thickness thin. An outer circumference of edge 1b is bonded to frame 7, and an inner circumference of main body 1a of the diaphragm is bonded to voice coil 6.
Voice coil 6 is held by suspension 8 in such a manner that coil 6a, which is formed at a lower end of voice coil 6, is kept in magnetic gap 5a. 
An outer circumference of suspension 8 is bonded to frame 7, and an inner circumference thereof is bonded to voice coil 6. Dust cap 9 prevents a foreign body from entering into magnetic circuit 5.
According to the loudspeaker constructed above, an audio signal is input from the outside (not shown) to coil 6a of voice coil 6, whereby voice coil 6 moves vertically by Fleming's left-hand rule based on the audio signal. Then diaphragm 1 moves vertically, so that a sound is emitted.
This kind of loudspeaker is disclosed in Unexamined Japanese Patent Publication No. H8-149594.
Recently, high sound quality has been required for various acoustic devices, and diaphragm 1 has been required to be lighter for improving sound pressure. However, the following problems may occur by merely reducing a thickness of diaphragm 1 or using material having a low density for reducing weight. In a word, because an elastic modulus of the diaphragm decreases, divided resonance tends to occur at the diaphragm. As a result, a sound-pressure-frequency characteristic extremely deteriorates.
In the conventional loudspeaker mentioned above, it is proposed to form the diaphragm or rib by coinjection molding, however, its characteristic is required to be further improved.
The present invention is directed to solve the problems pointed out above and aims to provide a high quality diaphragm having an excellent sound-pressure-frequency characteristic and a loudspeaker using the diaphragm.